The present invention is in the technical field of radio communications. More particularly, the present invention is in the technical field of antenna design.
A dipole antenna is the simplest form of radio antenna. It consists of two electrically-conductive elements positioned such that passing radio waves cause a voltage differential between the two elements.
In the case of radio signal reception, this voltage differential, however small, is passed through a feed line to a radio frequency (RF) receiver where it is converted into audio, video, text or other content.
In the case of transmission, an oscillating voltage differential is generated by a radio transmitter and passed to the antenna by way of a feed line, creating radio waves in space.
Dipole antennas may be “balanced”, having two elements of equal lengths with a feed point in the center. Or they may be “end-fed” with one element making up nearly the entire length of the antenna, with the feed point at or near the very end of the configuration. In reality, this type of antenna may be constructed with the feed point at any point between the end points with an impedance matching transformer used just below the feed point.
This type of antenna may be linear, with the two elements occupying a single line in space (or nearly a single line). Or they may be placed in an “inverted V” configuration with the feed point elevated above the two end points. Indeed, there are many serviceable configurations of a dipole antenna.
Changes to the position of the feed point and/or the configuration of the elements may cause a change or imbalance in the overall electrical impedance of the antenna. (Impedance is a combination of inductance, capacitance, and resistance and is measured in Ohms, signified by the capital Greek letter, Omega.)
Differences between the impedance of the antenna and the required impedance of the attached radio device are mitigated by the use of an “impedance matching transformer”.
A dipole antenna of a given overall length is most effective at receiving or emitting radio waves whose wavelength is TWICE the overall length of the antenna (with a correction applied for different element materials and insulations). In the case of reception, this ratio of antenna length to wavelength produces the maximum voltage differential across the elements. Conversely, during transmission, this ratio produces the maximum amplitude of radio wave for a given voltage differential applied to the antenna.
The impedance of a fixed-length dipole antenna changes across the frequency spectrum. The impedance of an antenna of length L may be 50 Ohms at 14.050 MHz, but may be 5 Ohms at 14.450 MHz.
Likewise, the standing wave ratio (SWR) of an given-length antenna will change across the frequency spectrum, changing the antenna efficiency as frequency changes while length is held constant.
Therefore, for a given frequency or frequency band, a specific overall length of dipole antenna is highly preferable for maximum efficiency.
An external antenna tuner may be used to compensate for an antenna that is not ideally matched to the frequency in use, however, this tuner cannot make an inefficient antenna ideally efficient, it can only make the antenna usable (such as not to damage radio equipment).
Ideally, for maximum radiating efficiency, with or without the use of an antenna tuner, the overall length of the dipole antenna is at or near ½ the wavelength of the frequency in use.
Traditionally, there have been two methods for creating dipole antennas suitable for multiple bands: telescoping and the use of traps.
Telescoping antenna elements are highly familiar in the form of the typical home TV rabbit-ear antennas. This device, however, becomes cumbersome and difficult to sustain at longer wavelengths—imagine a rabbit-ear antenna where each element telescopes out to 10 meters.
For longer HF applications, traps have been traditionally used. A trap is essentially a small inductor-capacitor circuit which has a fix resonant frequency which falls between the two bands it separates. The result is a fixed-length antenna which approaches maximum efficiency on multiple bands. (The costs of this solution are financial, inconvenience and in weight.)